Page 10 - May-June2019 Vol37 No1

METRO NASHVILLE By Mike Bernard, P.E.
Safer Disinfection for Water System

After losing one of their water treatment Left: Removal of last gaseous chlorine
plants to the floodwaters of May 2010, railcar from the Omohundro WTP. This
Nashville’s Metro Water Services (MWS), allowed MWS to feed bulk system
decided that it was time to move to an hypochlorite while the contractor
inherently safer disinfection system than demolished the gaseous chlorine equipment
gaseous chlorine. and installed the new on-site generation
equipment.
Gaseous chlorine had provided safe Below: Completed renovation of chlorine
water to Nashville residents and visitors room. Gaseous chlorine cylinders were
for nearly 100 years at the Omohundro removed and three onsite sodium
Water Treatment Plant (OMO), and for hypochlorite generators were installed
almost 40 years at the K.R. Harrington in the existing building at both water
Water Treatment Plant (KRH). However, treatment plants.
the risks of transporting the chemical by
both 90-ton rail car and by trucks through Above: Completed solution
the middle of Music City was simply too storage tank containment area
dangerous to continue. The department’s at K.R. Harrington WTP.
leadership already had decided that it Left: Completed metering pump
was time to remove the inhalation hazard room at K.R. Harrington. Room
from the bustling city (and the natural was repurposed for feeding
disaster simply advanced their timetable). sodium hypochlorite solution to
Alternatives Considered all the plant’s disinfectant feed
points.
Their evaluation of alternatives
identified two primary candidates: the use
of bulk sodium hypochlorite or on-site
generated sodium hypochlorite. During
and immediately after the flooding,
however, chemical deliveries to the two
plants were impossible.

Additionally, the city’s leaders did
not favor transporting one dangerous
chemical through the busy streets of
Nashville as a replacement for another
dangerous chemical. Thus the on-site
generation (OSG) of sodium hypochlorite
rose to the top of the list as the most
advantageous alternative for Metro Water.

The conversion from gaseous chlorine
to OSG was a challenge. Both 90-million-
gallon-per-day facilities were limited
on available space, and most of that
remaining space already had been
designated for future projects. In order
to make the conversion while preserving
real estate, a combination of new and
repurposed facilities would be necessary.
Phased Approach

Beginning in 2011, the engineering
teams devised a phased approach that
entailed the conversion of each facility
first to diluted bulk sodium hypochlorite.
Even these phases were staggered so that
the two facilities were not going through
the transition at the same time. This
provided another level of comfort and

10 TPW May/June 2019